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Transcript
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
495
Adaptive Modulation Control for Multiple-Phase Voltage Regulators
Shangyang Xiao, Weihong Qiu, Greg Miller, Thomas X. Wu, Senior Member, IEEE, and Issa Batarseh, Fellow, IEEE
Abstract—This letter presents an adaptive modulation control
(AMC) method which has proven to be effective in achieving high
bandwidth designs as well as stabilizing the control loop during
large load transients. The AMC adaptively adjusts control bandwidth by changing the modulation gain, depending on different
load conditions. With the AMC, a multiphase voltage regulator can
be designed with an aggressively high bandwidth, while in heavy
load transients where the loop could be potentially unstable, the
bandwidth is lowered. Therefore, the AMC provides an optimal
means for robust high-bandwidth design with excellent transient
performance. It is a type of non-linear control designed for, but
not limited to, multiphase voltage regulators with droop control.
Detailed analysis of the operational mechanism is presented. Experiment results are included to verify the analysis.
Index Terms—Adaptive modulation control (AMC), bandwidth,
multiphase voltage regulator (VR), transient response.
I. INTRODUCTION
N
EXT generation microprocessor (Vcore) requirements
for high-current slew rates and fast transient response,
together with low output voltages, have posed great challenges
on voltage regulator (VR) design. For today’s microprocessors,
the operational current may exceed 100 A with voltages in the
1 to 1.5 V range, while the slew rate of the transient current
can be over 1 A/ns [1]–[3]. Much effort has been made, such as
application of novel topologies, advanced devices, and control
schemes, to meet the stringent Vcore specifications for transient
response [4]–[14]. Droop control, also referred to as adaptive
voltage positioning (AVP) in [1]–[3], is popularly implemented
to achieve a cost-effective voltage regulation. With droop
control, the output voltage will droop when the load current
increases, resulting in improved voltage tolerance. As a result,
droop control allows more voltage deviation than voltage-mode
control; therefore a significant number of the output capacitors
can be saved for the same output voltage specifications.
It is desired to push the bandwidth as high as possible to
achieve a fast transient response. However, the maximum
system bandwidth is limited by the switching frequency. Typically the transient response obtained from a system with higher
switching frequency is better than that with lower switching frequency. For multiphase voltage regulators used in low-voltage
applications, the effective frequency of the output voltage ripple
is equal to the product of the phase number and phase-switching
frequency, as shown in Fig. 1(a). Based on conventional loop
analysis, the bandwidth of multiphase voltage regulators could
be pushed close to the phase-switching frequency.
Manuscript received March 22, 2007; revised June 8, 2007. Recommended
for publication by Associate Editor R. Zane.
S. Xiao, W. Qiu, and G. Miller are with the Intersil Corporation, Milpitas,
CA 95035 USA (e-mail: [email protected]).
T. X. Wu and I. Batarseh are with the School of Electrical and Computer
Engineering University of Central Florida, Orlando, FL 32816 USA.
Digital Object Identifier 10.1109/TPEL.2007.912947
Fig. 1. Interleaved operation for multiphase voltage regulators. (a) Interleaved
operation waveforms. (b) Bode plot of power stage.
For some advanced control schemes like adaptive phase alignment (APA) in [15], however, all phases are turned on simultaneously to increase the current slew rate during large load-insertion transients. In this case, the effective frequency of the
output ripple will be reduced to the phase-switching frequency,
as shown in Fig. 2(a). In frequency domain, a multiphase VR
with APA control during transients shows a different gain-phase
relationship from a regular multiphase VR. For a regular multiphase VR, the abrupt gain drop and phase delay caused by
the aliasing effect [16] occurs at phase number times the phaseswitching frequency, as shown in Fig. 1(b). For a VR in simultaneous operation, however, the gain and phase dips occur at
the phase frequency, as shown in Fig. 2(b). It behaves like a
single-phase voltage regulator. With a high bandwidth design,
this can be disastrous if care is not taken: sideband components
induced by the aliasing effect can result in system instability
0885-8993/$25.00 © 2007 IEEE
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496
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
Fig. 3. Simplified model for voltage regulator with droop control.
Fig. 2. Simultaneous operation for multiphase voltage regulators. (a) Simultaneous operation waveforms. (b) Bode plot of power stage.
[16]. Motivated by the demand for a control method which is
able to facilitate a high-bandwidth design as well as stabilize
the loop during large load transients, adaptive modulation control (AMC) is proposed in this letter.
II. MECHANISM OF ADAPTIVE MODULATION CONTROL
The block diagram of a typical voltage regulator with droop
control is shown in Fig. 3. The inductor current, , is sensed and
being the current sense gain.
fed to the error amplifier, with
, flows through
and develops a
This sensed current,
.
voltage droop across
Based on the simplified model for multiphase VRs with droop
control in [1], the voltage loop gain can be expressed by
(1)
where
is the current sense gain,
is the input voltage,
and
are the power stage parameters, and
are the compensation parameters.
Fig. 4. Simplified Bode plot of the loop gain with droop control [1].
is the modulation gain, which is inversely proportional to the
(referred to GND) of the ramp signal.
peak-peak voltage
The simplified Bode plot of droop control is illustrated in
Fig. 4. It is found that there exists a flat range between the
[3]. Therefore, the bandwidth can
ESR zero and the pole
be adjusted by varying the pole frequency. Since the pole
is determined by the output inductance, modulation gain and
other parameters, a change of the modulation gain will have
a direct impact on the system bandwidth. Adaptive modulation control is proposed by this motivation. The concept is to
design an aggressively high bandwidth for steady-state operation, while decreasing the bandwidth by reducing the modulation gain when phases are on simultaneously. If the modulation
will be pulled to a lower frequency, thus
gain is reduced,
reducing the high-frequency gain while keeping the low-frequency gain unchanged, as shown in Fig. 4. For a multiphase VR
with droop control, the decrease of high-frequency gain means a
decrease of bandwidth and possibly an increase of phase margin.
During a large load transient, a corresponding large perturbation
will be introduced to the output. This large perturbation in time
domain can be transformed into large high-frequency components in the frequency domain. Since the feedback control loop
functions as a low-pass filter, these high-frequency components
and their associated sideband frequencies can not be damped if
the bandwidth is too high [16]. With AMC, the loop gain at a
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
497
Fig. 6. Circuit implementation of AMC.
Fig. 5. Operational waveforms of AMC.
high frequency range is reduced to attenuate the high-frequency
components in large load-transient events.
Time-domain investigation of AMC leads to the discovery
that AMC shows great advantages in removing undesired
voltage spikes during large load-insertion transients. Fig. 5
shows the operational waveform comparisons of a two-phase
VR with traditional trailing edge modulation (TEM) and with
AMC for a load-insertion case. The TEM has a fixed ramp
signal RAMP1 (solid line), while the AMC has a flexible
ramp signal RAMP2 (dashed line). For traditional TEM, the
clock (CLK) signal initiates the pulsewidth modulation (PWM)
turn-on, while the PWM is turned off as the RAMP1 signal
intersects the output voltage of the error amplifier (COMP).
which can
The total inductor current is charged at slew rate
be expressed as
(2)
and
represent output voltage and phase inducwhere
tance, respectively. And N is the phase number. When APA [15]
is triggered, the system turns on all PWMs simultaneously. The
output inductance is equivalent to N inductors in parallel. Therefore, the sum of all inductor currents has approximately N times
the slew rate of that of a single inductor, i.e.,
(3)
where
represents the inductor current slew rate for simulis
taneous operation. For low-voltage applications where
is approximately
times
. In this
much less than
case, the effective charging time is reduced by a factor of .
As a result, the PWM turn-off of the simultaneous operation is
expected to be sooner. However, due to the fixed ramp signal
of the traditional modulator, the turn-off timing of PWMs
sees no change, even though all phases are on simultaneously
under large load-insertion events. Consequently, significant
turn-off delay is introduced during load-insertion transients.
This turn-off delay causes extra energy stored in the output
inductors to continue charging the output capacitors, resulting
in undesirable results, such as ring-back and longer settle-down
time, etc. With AMC, the optimal slew rate of the ramp signal
is adjusted, based on the number of on phases, as shown in the
simplified circuit implementation in Fig. 6. If PWM pulses of
both phases are on simultaneously, the ramp signal rises with
twice the slew rate of the ramp in normal interleaved operation,
as the dashed line shows in Fig. 5. In this manner, the modulator
Fig. 7. Pspice simulations for VRs without and with AMC, using LGA775
Socket and VTT Tool simulation model provided by Intel. System parameters: V
12 V, V
1.2 V, I
50 A, L
310 nH, C
5
680 F 12
16 F. (a) Simulated transient response without AMC. (b) Simulated transient response with AMC.
=
+ 2
=
=
=
= 2
gain decreases by half under a transient event. And the PWM
is terminated at t1 rather than t2, resulting in a reduced turn-off
delay. Since AMC is activated by large load transients, it will
not sacrifice steady-state stability. On the contrary, compared
with normal design of the same bandwidth as AMC, during
large load transients AMC can achieve faster transient response
due to its higher steady-state bandwidth.
Fig. 7(a) and (b) shows simulation results for a three-phase
droop-control VR without and with the AMC scheme. The inductor currents, PWMs, and output voltages are marked as
, PWM1, PWM2, and
on the waveforms, respectively.
During a large load transient, all phases are on simultaneously
for fast transient response. The first voltage spike is determined
by ESR and ESL. Following the first spike, for traditional
trailing-edge PWM modulation control in Fig. 7(a), there is significant voltage ring-back due to extra energy in the inductors
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498
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
Fig. 9. Experimental measurement of loop gain and phase for VR with AMC
enabled and disabled.
IV. CONCLUSION
Fig. 8. Experimental result comparison, using Intel LGA775 VTT tool. System
12 V, V
1.2 V, I
95 A, L
315 nH, C
parameters: V
5 560 F OSCON 12 22 F MLCC 6 10 F MLCC. (a) Transient
response without AMC. (b) Transient response with AMC.
2
=
+ 2
=
=
+ 2
=
=
during the transient event. AMC removes the turn-off delay by
increasing the ramp slew rate; therefore the ring-back voltage
is avoided and the system can settle down in a short time, as
shown in Fig. 7(b).
III. EXPERIMENT VERIFICATION
A 300 kHz four-phase VR designed with very high bandwidth and excellent transient response is built to verify AMC.
The measured transient responses in time domain for the VRs
without and with AMC are shown in Fig. 8(a) and (b). The
and the lower
upper waveforms represent the output voltages
. From the experiment
waveforms represent load currents
comparison, it is found that excellent transient performance is
achieved for the voltage regulator with AMC. The voltage spikes
are successfully suppressed under a large load transition.
Measured Bode plots of the closed loop system shown in
Fig. 9 illustrate how AMC adaptively adjusts the control bandwidth. The grey curves represent gain and phase of the loop
during steady state (AMC is not triggered), while the black
curves are gain and phase of the loop with AMC enabled. When
AMC is not triggered, the bandwidth is about 420 kHz and the
phase margin is 61 . During a large load transient, AMC pulls
down the modulation gain and drops the bandwidth to 94 kHz.
Hence, the phase margin is boosted to 139 .
Traditional PWM modulation schemes fail to achieve a robust
high-bandwidth design and remove voltage ring-back during
large load transients. The proposed AMC provides an effective solution by adaptively changing the modulation gain during
large load transients. With AMC, it is possible to push the bandwidth above the switching frequency for multiphase VR applications with robust operation under all conditions. Simulation and experimental results have verified the proposed AMC
scheme and corresponding analysis.
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
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